2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/7107A strong differential magnification over the face of the source star passing the caustic created by a binary lens star allows to measure its radial intensity profile with an angular resolution of 20-60 nas from broad-band photometric observations for ~15 stars per year and to study its chemical composition from time-resolved high-resolution spectroscopy for ~2 stars per year.3 pages in LaTeX2e, 2 EPS-figures embedded, uses 'ws-procs975x65.cls' and 'rotating_pr.sty' (supplied). To appear in the Proceedings of the Xth Marcel Grossmann Meeting on General Relativity (Rio de Janeiro, Brasil, 20-26 July 2003), ed. S. Perez Bergliaffa, M. Novello & R. Ruffini (World Scientific, Singapore)Astrophysicstext